The objective of this project is to investigate age-dependent changes in mitochondrial metabolism in the mammalian brain. We have observed that the oxidation of labeled glucose and 3-hydroxybutyrate was diminished by about 35% in cerebral cortex slices from 2 year-old rats compared to 3 month-old animals. A significant reduction in ketone body-metabolizing enzymes were also observed in whole brain homogenates of 2 year-old rats. Further, age-dependent changes are also observed in the metabolism of pyruvate and 3-hydroxybutyrate by non-synaptic and synaptic mitochondria from brains of aging rats. These findings are supported by the changes in the oxidation of labeled substrates by both mitochondrial populations from old rats. although age-dependent changes in the levels of ketone body-oxidizing enzymes were observed in both mitochondrial populations, the activity of pyruvate dehydrogenase was not altered in these preparations. These findings show that specific alterations in oxidative metabolism occur in non-synaptic and synaptic mitochondria of aging rats. The findings also suggest that in addition to alterations in enzyme activities, permeability of anions across the inner membrane may be altered in brain mitochondria from senescent animals.